Drawing of synthetic fibers at a high draw ratio has been performed in order to increase strength and/or elastic modulus of the synthetic fibers. For example, in order to increase initial elastic modulus of a polyester fiber, JP Examined Patent Publication No. H06-4704 describes a method including: a first step of taking up (winding) an undrawn fiber obtained by melt spinning; a second step of subsequently drawing the fiber while heating the fiber by a preliminary heating roller, where the second step is repeated twice; and a third step of heat treating the fiber. As a result, a highly elastic polyester fiber having low shrinkage property can be obtained. In a method of JP Examined Patent Publication No. H07-33610, in order to obtain a polyester fiber having a high modulus, excellent dimensional stability, and high toughness, without passing a winding step after the spinning, an undrawn fiber obtained by melt spinning is continuously subjected to drawing and subsequent heat treatment, where the drawing is performed by repeating a step of preheating the fiber by a roller heated at 80 to 110° C. and drawing the fiber for three times. As a result, a polyester fiber having a high modulus, excellent dimensional stability, and high toughness can be obtained.
Ordinary rollers are used for the drawing in the above-exemplified method. As an example of using a taper shaped roller as a part of a drawing device, JP Laid-open Patent Publication No. S50-83516 describes a heat drawing between Nelson type feed rollers and draw rollers. Such a method includes a problem in that yarns on the preheated feed roller may be loosened due to thermal elongation, resulting in swaying of yarns on the roller. In order to solve this problem, JP Laid-open Patent Publication No. S50-83516 describes a method in which one of the feed rollers is made to have a tapered shape so as to eliminate the looseness of the yarns.
As another example of using a taper-shaped roller as a part of a drawing device, JP Examined Patent Publication No. S47-42842 describes preliminary drawing using a taper-shaped roller.